1. Field of the Invention
This invention relates to an optical disk device for making storage, reproduction and erasure of an optical disk having an information recording layer, such as a compact disk (CD), a digital versatile disk (DVD), a laser disk (LD), a magneto-optic disk (MO), a minidisk (MD), etc.
2. Description of the Related Art
As shown in FIG. 5, in an optical disk device 10 for recording information on an optical recording medium such as an optical disk 101 or reproducing the recorded information, a laser beam irradiated to an objective lens 105 arranged in the vicinity of the optical disk 101 is focused onto the optical disk 101. Information tracks 102 are formed on the optical disk 101. The laser beam reflected from the information track 102 of the optical disk 101 being rotated is read by a signal detecting system to reproduce the information. Since eccentricity occurs inevitably while the optical disk 101 is rotated, tracking control 103 and focusing control 104 are performed so that the laser beam accurately tracks the information tracks 102.
The conventional mechanism for the tracking control and focusing control in the optical disk device is disclosed in JP-A-5-28526, JP-A-5-210855, JP-A-2-23112, JP-A-6-325397 and JP-A-11-250489.
In an example of the conventional tracking control, a tracking actuator (not shown) shifts the objective lens 105 in a direction orthogonal to the optical axis of the laser beam and also drives a movable mirror 107 immediately in front of the objective lens 105 so that the focal point position (laser spot) of the laser beam is shifted in a radial direction of the optical disk, thereby applying alignment of the focal point position, i.e. tracking servo.
As the movable mirror 107, a micro electromechanical system 700 as shown in FIG. 6 can be used. The movable mirror 107 using the micro electromechanical system 700 includes a substrate 108 made of a square silicon wafer (having a thickness of 10 μm to 100 μm), a mirror section 108a formed on the substrate 108, two permanent magnets 109 to lie the substrate 108 therebetween, rotary shafts 110a and 110b formed on opposite sides of the substrate 108 respectively and making the substrate 108 rotatable within a predetermined angle, and a coil section 108c formed on the substrate 108. When a current passes through the coil section 108c, a rotational torque is generated by Lorentz force F (F=η×i×B×L) based on the magnetic field B formed by the permanent magnets 109 and the current i crossing the magnetic field B so that the substrate 108 is rotated about the rotary shafts 110a and 110b. Thus, the focal point of the laser beam is controlled. Here, η represents the efficiency and L represents the distance over which the current I flows perpendicularly to the direction of the magnetic field B.
As shown in FIG. 5, the movable mirror 107 is located at a position where the laser beam emitted from a laser light source 110 becomes parallel light, i.e. ahead of a collimator lens 111. The width (width of light rays) of the laser beam being the parallel light is wider than that of the laser beam immediately after emitted from the laser light source 110. The size of the movable mirror 107 need to be larger than the width of the laser beam being the parallel light to reflect the parallel light.
As a condition of the radius φ (φ=0.82·λ·NA) of the permissible circle of confusion at a depth of focus of the laser beam, the wave aberration (RMS wave aberration) of the focused laser beam λrms need generally be included within 1/14 times of the wavelength λ of the laser beam ( 1/14 λrms: “Marechal criterion”) to read the information tracks 102 of the optical disk 101 accurately using the focused laser beam. The numerical aperture NA is represented by the product of the sinusoidal value of the angle formed by the optical axis of the laser beam and the periphery of the laser beam and the refractive index (n×sin α).
Assuming the width of the laser beam being the parallel light is about 5 mm, in order to deflect this laser beam by the movable mirror 107 using the micro electromechanical system 700, the movable mirror 107 need be formed in the shape of a square in plan view, which is about 7 mm long and 5 mm wide. Since the substrate 108 of the movable mirror 107 is made of a silicon wafer which is 10 to 100 μm thick, the substrate 108 is distorted in a wave shape if the substrate is formed in a square which is about 7 mm long and 5 mm wide. Correspondingly, the mirror section 108a formed on the substrate 108 is also distorted, thus peaks and valleys are formed on the surface of the mirror section 108a. Therefore, owing to differences between the peaks and the valley, concavity and convexity having a size of about 0.1 mm appear on the surface of the mirror section 108a. As a result, when the surface of the mirror section 108a is irradiated with the laser beam being the parallel light of 5 mm width, the laser beam reflected from the mirror section 108a produces large wave aberration. The disorder in the wave aberration made it difficult to satisfy the condition of the effective wave aberration in the “Marechal criterion”. Accordingly, a read error may occur when the information tracks 102 of the optical disk 101 are read using the focused laser beam. Thus, using the movable mirror 107 provided with the micro electromechanical system 700 is problematic.
Further, the conventional tracking control is difficult to increase the high-order resonance frequency so that it cannot enhance the gain of the tracking control (cannot increase the gain intersection point frequency) and is difficult to deal with the high speed rotation of the disk. Further, since the amplitude of an error signal which can be suppressed by the tracking control is within a range of about ±0.1 μm, the tracking of the laser spot for the information tracks of the optical disk should be improved.